1. Field of the Invention
The present invention relates to a photoelectric conversion element, or a photovoltaic element which converts light to electric energy, and more particularly to a solar cell having excellent mass producibility and high reliability, and a fabrication method of the solar cell.
2. Related Background Art
Solar cells which are photoelectric conversion elements for converting solar light to electric energy have been considered a substitute energy source for oil. Examples of solar cells include single crystal silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon-type solar cells.
The fabrication method of the single crystal silicon solar cell relies on a semiconductor process with a high production cost. Since single crystal silicon has a small light absorption coefficient due to its indirect band gap transition, a solar cell of single crystal silicon must be at least 50 microns thick to absorb the incident solar light, resulting in a high cost of material; and with the band gap being about 1.1eV, the short wavelength components having a wavelength of not more than about 500 nm within the solar light spectra cannot be effectively used because of the problem of the surface recombination or wasted energy at or below the band gap.
Polycrystalline silicon also has an indirect band gap transition problem even though it can have a reduced production cost, as compared with the single crystal silicon, so that the thickness of the solar cell cannot be reduced. Moreover, polycrystalline silicon has also other problems such as grain boundaries.
Meanwhile, as one candidate for substituting solar cells for utility generated electric power, the electric power necessary for an individual house is about 3KW. On the other hand, since the solar energy flux is 1Kw/m at peak, solar cells occupying an area as large as are necessary if the conversion efficiency is 10%. Accordingly, it is necessary to fabricate a solar cell module as large as possible in order to use them as the electric power source.
However, with single crystal silicon, a large wafer cannot be fabricated, so that realization of a large area cell is difficult, whereby it is requisite to provide wiring to connect unit elements in series or parallel in order to obtain a large power output. Further, since the single crystal silicon is mechanically weak, an expensive encapsulation made of glass or polymer resin, or an aluminum frame is necessary in order to protect the solar cell from mechanical damage caused by various weather conditions when it is used outdoors. Consequently, there is a problem that the production cost per unit amount of power generation is higher than the existing power generation methods.
In view of these considerations, studies have been made of the amorphous silicon-type solar cell which can be produced at a low cost and with a large area.
FIGS. 7A to 7C show typical examples of conventional solar cells. In the figures, 700 is a solar cell main body, 702 is a lower electrode, 703 is an n layer, 704 is an i layer, 705 is a player, 706 is a transparent electrode, 707 is a passivation layer, 708 is a collector electrode, and 709 is a bus bar.
The structure of an amorphous silicon-type solar cell is generally obtained by laminating the transparent electrode 706 for the reduction of surface resistance on one or more pairs of semiconductor junctions composed of the thin film p layer 705, the i layer 704, and the n layer 703 provided on the substrate 701, thereafter depositing the collector electrode 708 made of a relatively thin metal layer for collecting the electric current, and further depositing an electrode made of a relatively thick metal called the bus bar 709 for the collection of the current collected by the collector electrode 708.
The problem encountered when such an amorphous silicon-type solar cell is deposited on a large substrate, e.g. about 30 cm square, or continuously deposited on a long substrate with a roll-to-roll method, is that pin holes or other defects occur in the fabrication process, causing a shunt or short circuit, so that the conversion efficiency of solar cell may decrease. This is due to the fact that since the substrate surface is not a completely smooth plane, and may contain some flaws, dimples or spike-like projections, or may be provided with an irregular back reflector for the purpose of reflecting the light impinging on the substrate, a thin film semiconductor layer only several hundreds .ANG. thick such as a the p or n layer cannot cover such a surface. In addition, pin holes may occur due to the presence of dust particles during the film formation. There is no problem if a normal semiconductor junction exists between the lower and upper electrodes, but if the semiconductor junction is lost due to a defect as mentioned above so that the upper and lower electrodes are directly contacted with each other, or if a spike from the substrate is placed in contact with the upper electrode, or if the semiconductor layer becomes of low resistance even though not completely lost, the electric current produced by the light will flow into such defective portion, whereby the photogenerated current cannot be effectively collected. When the defective position is away from the collector electrode or the bus bar, the resistance of the current flowing into the defective portion is great, with relatively small current losses, but conversely, if it is under the collector electrode or the bus bar, the lost current is greater due to such defect.
Owing to the opacity of the collector electrode and the bus bar, the effective area of the solar cell is reduced. .In order to reduce the area of the collector electrode and the bus bar, and output the electric current effectively, it is desirable to reduce the specific resistances of the collector electrode and the bus bar and to increase the cross-section of the electrode. Accordingly, a material having a small resistivity such as silver or copper is suitable as the electrode material. For example, the resistivity of silver is 1.62.times.10.sup.-6 .OMEGA.cm and the resistivity of copper is 1.72.times.10.sup.-6 .OMEGA.cm, while aluminum has a resistivity of 2.75.times.10.sup.-6 .OMEGA.cm and zinc has a resistivity of 5.9.times.10.sup.-6 .OMEGA.cm.
The conventional methods for forming such an electrode are any one of the vapor deposition method, the plating method, or the printing method if the solar cell is of crystal type. With the vapor deposition method, a good quality metal can be deposited in excellent ohmic contact with the semiconductor, but there is a disadvantage that the deposition rate is slow, the throughput is low due to the vacuum process, and a mask is required to form a specific pattern. With the plating method, electroless plating of Ni is generally conducted, but has a problem that exfoliation easily occurs and a mask is required. The printing method has an advantage that automation is most easily realized and the mass producibility is high, one method being adopted in which Ag paste is screen printed, sintered at high temperature, and contacted. To decrease the resistance, it is contemplated that a plating or soldering coat be applied to the printed electrode.
In the amorphous silicon-type solar cell, all the above methods have been investigated, but practically, the printing method has been implemented because of its superior mass producibility. However, the amorphous silicon cannot be sintered, unlike the above-mentioned crystal type, so that a highly resistive electrode results. That is, since the conductive paste of silver contains a polymer resin as the binder, the resistivity is about 4.times.10.sup.-5 .OMEGA.cm, which is one digit higher than that of the pure silver. Accordingly, in order to reduce the resistance without changing the area of collector electrode, it is desirable to increase the thickness of the electrode. However, with the increased thickness, it is necessary to raise the viscosity of the conductive paste; however there is a limit because the screen may become clogged. Therefore, the thickness of practical electrodes fabricated by the screen printing is in a range of 10 .mu.m to 20 .mu.m. Accordingly, the collector electrodes fabricated by the screen printing tended to be wider to decrease the resistance.
Thus, in current practice, when screen printing is used, small electrodes cannot be formed, resulting in a great loss of effective area.
Meanwhile, as a countermeasure against shunt or short circuits as described above, it is effective for preventing the decrease in the conversion efficiency that only the defective part be selectively covered with an insulating material or material having an adequately high resistance to increase the contact resistance with the transparent electrode, the collector electrode, or the bus bar. Selective treatment of only the defective part is the best method, but requires a complex process.
Other than such a selective insulation, there are known methods in which the portion under the bus bar having the greatest influence is insulated with polymer or oxide as disclosed in U.S. Pat. No. 4,590,327, and shown in FIG. 7C, in which the portion corresponding to the pattern of the bus bar and collector electrode between the semiconductor layer and the transparent electrode is provided with a material having a low electrical conductivity of 3000 .OMEGA./.quadrature. or greater, or an insulating material as disclosed in U.S. Pat. Nos. 4,633,033 and 4,633,034.
However, since in the conventional method the insulating material or highly resistive material is disposed in accordance with the pattern of the bus bar and collector electrode, it is necessary that the insulating material having substantially the same shape be patterned to correspond to the bus bar, thus resulting in an impractical process due to the necessity of a patterning process. Also, since the bus bar is laminated on the insulating material or highly resistive material, the adhesion strength is weak, thus requiring an adhesive for the bus bar.
Also, since in the conventional method the collector electrode itself is provided with the resistance, instead of providing the insulating material under the collector electrode, the series resistance of the solar cell is great, resulting in a low conversion efficiency. If only the collector electrode is fabricated by the normal method to avoid such drawback, there is a great possibility of causing a short circuit when the under portion of the collector electrode contains some defects.
Also, the solar cell is unstable to ambient conditions such as heat, light or humidity, even though the initial characteristic of solar cell module is sufficient. For this reason, there is a possibility that a short circuit might occur in actual use.
The plating method can be used as with the crystal system, and for example, in Japanese Patent Application Laid-Open No. 60-66426, there is disclosed a method of forming an electrode on the transparent electrode of the solar cell by the plating method. However, with this method, it is necessary to perform the patterning with the resist to protect portions other than the collector electrode from being plated. The plating method has associated therewith several problems due to the necessity of patterning, e.g. the fabrication process becomes complex, or if a thick plating membrane is formed on the transparent electrode, the solar cell may be adversely affected because the solar cell must be immersed in the acid or alkali plating bath for a long time, or the formed electrode is easily exfoliated due to its thick membrane. Moreover, in plating, a problem may arise that the transparent electrode is easily affected by hydrogen produced in a competing reaction.